Correct option is B
The correct answer is (b).
Nitrous acid (HNO2) is a mutagen that deaminates adenine, causing it to convert into hypoxanthine. Hypoxanthine pairs with cytosine instead of thymine, leading to an A-T to G-C transition. This type of mutation results from the base change where the normal adenine (A) is substituted by hypoxanthine, which pairs with cytosine (C), changing the original A-T pair to a G-C pair in the next DNA replication cycle. This is a well-known mechanism of transition mutation induced by chemical mutagens.
Incorrect options-
Option (a): UV radiation causes covalent bonding between adjacent thymines, producing thymine dimers that block replication and transcription but do not cause direct base pair transitions like G-C to A-T.
Option (c): Ethidium bromide causes insertion or deletion mutations due to its intercalation between base pairs but does not cause base substitution mutations.
Option (d): EMS is not a base analogue but an alkylating agent, which modifies bases chemically rather than mimicking them.
